CAUGHT IN THE ACT, SCIENTISTS WATCH SUPERNOVA EXPLODE

Scientists using NASA's Swift satellite and a combination of orbiting and ground-based observatories have for the first time caught a supernova in the act of exploding.

The event became visible on February 18. It was announced by NASA at the time as an unusual gamma-ray burst, about 25 times closer and 100 times longer than the typical gamma-ray burst. Careful, multi-wavelength analysis has now revealed exactly what took place.
The initial burst, in the form of a jet of high-energy X-rays, pierced through a doomed star from its core and sent out a warning within minutes that a supernova was imminent. The massive star then blew apart with observers collecting data throughout the event. The results are described in four papers appearing in the August 31 issue of Nature.

"Such details have never been revealed so early and so clearly," said Neil Gehrels, Swift principal investigator at NASA's Goddard Space Flight Center, Greenbelt, Md. "Normally, supernovae are discovered in optical wavelengths after the debris begins to clear and optical light breaks through, often days later."

A team led by Sergio Campana of the Italian National Institute for Astrophysics in Milan used Swift's three telescopes to observe the star fly apart. The scientists captured X-rays fading to ultraviolet and optical light, evidence of the shock wave from the explosion pushing exploded star-stuff into the surrounding medium.

A second team led by Alicia Soderberg, a graduate student at Caltech in Pasadena, Calif., observed the February event with the Very Long Array radio observatory and NASA's Chandra X-ray Observatory. Her team determined that the burst, now more properly described as an X-ray Flash, represents a type of explosion ten times more common than gamma-ray bursts.

Whereas gamma-ray bursts mark the birth of a black hole, X-ray flashes appear to signal the type of star explosion that leave behind a neutron star. Soderberg's group and an independent team led by Paolo Mazzali of the Max Planck Institute for Astrophysics in Germany postulate that the February 18 event might have led to a highly magnetic type of neutron star called a magnetar.

Remarkable details about the chemical composition of the star debris continue to be analyzed. Mazzali's group recommends a new supernova classification, called Type Ic/d, because of characteristics that differentiate the explosion from all known supernovae. This was the second-closest "gamma-ray burst" ever detected, about 440 million light-years away toward the constellation Aries.

A group led by Elena Pian of the Italian National Institute for Astrophysics also confirmed that the event, called GRB 060218 or XRF 060218, was tied to supernova called SN 2006aj a few days later. Because the event was so close, scientists hope to uncover what special star properties are needed--mass or magnetism, for example--to create a gamma-ray burst and black hole or X-ray flash and neutron star.

Launched in November 2004, Swift detects gamma-ray bursts and X-ray flashes and begins to relay their coordinates to the scientific community within seconds. This has led to a series of rapid follow-up observations by Chandra, the Hubble Space Telescope and other major observatories, which is slowly unraveling the mystery of star explosions.

Goddard manages Swift, a NASA mission with participation of the Italian Space Agency and the Particle Physics and Astronomy Research Council in the United Kingdom.

For animation and a before-after image of the February 28 explosion, visit: